U.S. patent application number 10/485661 was filed with the patent office on 2004-09-09 for exhaust gas purifying apparatus, particulate filter and manufacturing method thereof.
Invention is credited to Hirota, Shinya, Itoh, Kazuhiro, Nakatani, Koichiro.
Application Number | 20040172929 10/485661 |
Document ID | / |
Family ID | 19071805 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040172929 |
Kind Code |
A1 |
Itoh, Kazuhiro ; et
al. |
September 9, 2004 |
Exhaust gas purifying apparatus, particulate filter and
manufacturing method thereof
Abstract
This invention relates to an exhaust gas purifying apparatus
having a particulate filter for collecting particulates in exhaust
gas. This particulate filter (22) contains partition walls defining
paths (50, 51) in which exhaust gas flows. This partition wall is
formed of porous material. This particulate filter (22) is created
by gathering tips of the partition walls and then baking with the
adjacent partition walls being in contact with each other. The
adjacent partition walls are bonded together at a predetermined
bonding strength if the partition walls are baked such that they
are in contact. According to this invention, the end portion of the
particulate filter (22) has a higher strength than the
predetermined bonding strength.
Inventors: |
Itoh, Kazuhiro;
(Mishima-shi, JP) ; Hirota, Shinya; (Susono-shi,
JP) ; Nakatani, Koichiro; (Susono-shi, JP) |
Correspondence
Address: |
Oliff & Berridge
PO Box 19928
Alexandria
VA
22320
US
|
Family ID: |
19071805 |
Appl. No.: |
10/485661 |
Filed: |
February 3, 2004 |
PCT Filed: |
August 7, 2002 |
PCT NO: |
PCT/IB02/03114 |
Current U.S.
Class: |
55/523 ;
422/177 |
Current CPC
Class: |
B01D 46/2451 20130101;
F01N 3/085 20130101; F01N 3/0821 20130101; B01D 46/2425 20130101;
F01N 3/0222 20130101; Y10S 55/30 20130101; B01D 46/24492 20210801;
B28B 11/006 20130101; F01N 2240/20 20130101; B01D 46/244 20130101;
F01N 2570/16 20130101; Y10S 55/05 20130101; F01N 2510/0682
20130101; B01D 46/2498 20210801; B01D 46/2459 20130101; F01N
2260/18 20130101; B28B 3/269 20130101; B01D 46/2429 20130101; Y10S
55/10 20130101; B01D 46/2474 20130101; B01D 46/2486 20210801; F01N
3/0842 20130101; B01D 46/84 20220101; B01D 2279/30 20130101 |
Class at
Publication: |
055/523 ;
422/177 |
International
Class: |
B01D 039/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 8, 2001 |
JP |
2001-241341 |
Claims
1. A particulate filter having: a partition wall made of porous
material for defining a path allowing exhaust gas to flow; and an
end portion in which an opening of the path is closed by a bonding
portion bonded together at a predetermined bonding strength when
tips of the partition walls gathered so that they contact each
other are baked, for collecting particulates in exhaust gas,
characterized by comprising a reinforcement member that reinforces
the bonding portion provided at the end portion by covering the
tapered walls of the partition walls at their end portions.
2. An exhaust gas purifying apparatus comprising the particulate
filter according to claim 1.
3. The exhaust gas purifying apparatus according to claim 2,
wherein the reinforcement member is a member made of porous
material, loaded with a substance capable of oxidizing the
particulates.
4. The exhaust gas purifying apparatus according to claim 2,
wherein an average pore diameter of the reinforcement member is
smaller than an average pore diameter of the partition wall.
5. The exhaust gas purifying apparatus according to anyone of
claims 2 to 4, wherein the adjacent partition walls at the end
portion are bonded together over a predetermined length from the
tip of the partition wall.
6. The exhaust gas purifying apparatus according to claim 5,
wherein the adjacent partition walls to be bonded together are
parallel over the predetermined length from the tip of the
partition wall.
7. The exhaust gas purifying apparatus according to anyone of
claims 2 to 6, wherein the tips of the adjacent partition walls are
bonded together through a predetermined contact area wherein a
contact area of the tips of the adjacent partition walls is
increased by increasing the contact area per unit area of the tips
at the bonding portion.
8. The exhaust gas purifying apparatus according to claim 7,
wherein by decreasing an average pore diameter of the bonding
portion, the contact area is increased.
9. The exhaust gas purifying apparatus according to claim 8,
wherein by loading the bonding portion with a substance capable of
oxidizing the particulates, the average pore diameter of the
bonding portion is decreased.
10. The exhaust gas purifying apparatus according to claim 9,
wherein the partition wall is loaded with a substance capable of
oxidizing the particulates and the amount of substance loaded on
the bonding portion is larger than the amount of the substance
loaded on a portion of the partition wall other than the bonding
portion.
11. The exhaust gas purifying apparatus according to anyone of
claims 2 to 6, wherein the bonding portion is loaded with a
substance capable of oxidizing the particulates.
12. The exhaust gas purifying apparatus according to anyone of
claims 2 to 11, wherein the particulate filter contains a plurality
of paths and in part of the paths, downstream end portions of the
partition walls defining the paths are gathered while in the
remaining paths, upstream end portions defining the paths are
gathered.
13. A manufacturing method of a particulate filter for collecting
particulates in exhaust gas, comprising: forming a preliminary
formed body having partition walls defining a path by extruding
porous material; closing the path of the preliminary formed body by
gathering an end portion of the partition wall of the preliminary
formed body so that tips of adjacent end portions are in contact
with each other; baking the preliminary formed body; and
reinforcing the closed portion in the path by covering the tapered
walls of the partition walls at their end portions.
14. The manufacturing method according to claim 13, wherein the
closed portion is reinforced by disposing a reinforcement member at
the end portion of the partition walls of the preliminary formed
body, gathering the tips of the partition walls of the preliminary
formed body together with the reinforcement member, closing the
path of the preliminary formed body by bringing the tips of the end
portions into contact with each other and baking the preliminary
formed body and the reinforcement member.
15. The manufacturing method according to claim 13, wherein after
the path of the preliminary formed body is closed, the closed
portion is reinforced by providing the end portion with the
reinforcement member.
16. The manufacturing method according to anyone of claims 13 to
15, wherein the closed portion is reinforced by loading the tip of
the partition wall with a substance capable of oxidizing
particulates.
17. A manufacturing method of the particulate filter for collecting
particulates in exhaust gas comprising: forming a preliminary
formed body having partition walls defining a path by extruding
porous material; closing the path of the preliminary formed body by
gathering an end portion of the partition wall of the preliminary
formed body so that tips of adjacent end portions are in contact
with each other; baking the preliminary formed body; and loading
the end portion of the partition wall closing the path of the
preliminary formed body with a substance capable of oxidizing
particulates.
18. The manufacturing method according to anyone of claims 13 to
17, further comprising a step of: reducing an average pore diameter
of the end portion of the partition wall closing the path in the
preliminary formed body between closing the path and baking the
preliminary formed body.
19. A particulate filter for collecting particulates in exhaust gas
having: a body portion formed with partition walls made of porous
material defining a path in which the exhaust gas flows; and an end
portion including a bonding portion bonded at a predetermined
bonding strength when tips of the adjacent partition walls are in
contact with each other and baked, characterized in that an amount
of substance capable of oxidizing the particulates loaded on the
end portion is larger than the amount of the substance loaded on a
portion of the partition wall other than: the: end portion.
20. The particulate filter according to claim 19, wherein the
substance capable of oxidizing the particulates is loaded on the
bonding portion only.
21. An exhaust gas purifying apparatus comprising: the particulate
filter according to claim 19 or 20.
22. An exhaust gas purifying apparatus comprising: a particulate
filter for collecting particulates in exhaust gas and a
reinforcement member provided at an end portion of a path of the
particulate filter, wherein the end portion of the path of the
particulate filter includes a bonding portion bonded together at a
predetermined bonding strength when tips of adjacent partition
walls formed of porous material defining the path are brought into
contact and baked, and wherein said reinforcement member covers the
tapered walls of the partition walls at their end portions.
23. A particulate filter for collecting particulates in exhaust gas
comprising: a particulate filter for collecting particulates in
exhaust gas wherein an end portion of a path of the particulate
filter including the bonding portion bonded at a predetermined
bonding strength when tips of the adjacent partition walls are in
contact with each other and baked, and an amount of substance
capable of oxidizing the particulates loaded on the bonding portion
is larger than the amount of the substance loaded on a portion of
the partition wall other than the end portion.
24. The particulate filter according to claim 23, wherein the
substance capable of oxidizing the particulates is loaded on the
bonding portion only.
25. A particulate filter for collecting particulates in exhaust gas
comprising: a body portion formed with partition walls made of
porous material defining a path in which the exhaust gas flows; an
end portion including a bonding portion bonded at a predetermined
bonding strength when tips of the adjacent partition walls are in
contact with each other and baked; and a reinforcement member that
reinforces the bonding portion provided at the end portion by
covering the tapered walls of the partition walls at their end
portions.
26. A particulate filter for collecting particulates in exhaust gas
comprising: a body portion formed with partition walls made of
porous material defining a path in which the exhaust gas flows; and
a bonding portion baked with tips of the adjacent partition walls
being in contact with each other, wherein an amount of substance
capable of oxidizing the particulates loaded on the bonding portion
is larger than the amount of the substance loaded on a portion of
the partition wall other than the end portion.
27. The particulate filter according to claim 26, wherein the
substance capable of oxidizing the particulates is loaded on the
bonding portion only.
Description
FIELD OF THE INVENTION
[0001] The invention relates to an exhaust gas purifying apparatus,
particulate filter and manufacturing method thereof.
BACKGROUND OF THE INVENTION
[0002] A particulate filter for collecting particulates in exhaust
gas emitted from an internal combustion engine has been disclosed
in published Japanese translation of PCT-application,
JP-T-8-508199. In this particulate filter, a honeycomb structure is
formed of porous material and some of a plurality of paths
(hereinafter referred to as filter paths) in this honeycomb
structure are closed at their upstream ends, while remaining filter
paths are closed at their downstream ends. Consequently, exhaust
gas flowing into the particulate filter always passes through
porous walls (hereinafter referred to as filter partition walls)
forming the filter paths and flows out of the particulate
filter.
[0003] In this particulate filter, since exhaust gas always passes
through the filter partition wall and after that, flows out of the
particulate filter, its particulate collection rate is higher than
the particulate collection rate of a particulate filter in which
exhaust gas only passes through the filter paths without passing
through the partition walls of the particulate filter.
[0004] In the particulate filter disclosed in the above described
publication, the filter path is closed by gathering the end
portions of the filter partition walls and bonding together these
end portions. Consequently, the exhaust gas flow-in opening in the
filter path is shaped in a funnel. If the exhaust gas flow-in
opening in the filter path is shaped in a funnel, exhaust gas flows
into the filter path smoothly without a turbulent flow. That is, no
turbulent flow is generated in exhaust gas when exhaust gas flows
into the filter path. Thus, pressure loss of the particulate filter
disclosed in the publication is low.
[0005] In the particulate filter of the above-described type, the
filter path is completely closed by gathering the end portions of
the filter partition walls such that the end portions are in
contact with each other and baking the end portions being in
contact with each other so as to bond together the end portions.
Consequently, the filter path is completely closed. However, when
the end portions that are in contact with each other are baked,
these end portions are separated due to an influence of thermal
expansion of the end portions and surrounding filter partition
walls, so that the filter path may not be completely closed.
DISCLOSURE OF THE INVENTION
[0006] It is an object of the invention to close the filter path at
its end portion securely in a particulate filter of the
above-described type.
[0007] A first aspect of the invention relates to an exhaust gas
purifying apparatus having a particulate filter for collecting
particulates in exhaust gas. This particulate filter has a
partition wall, which defines a path in which exhaust gas flows.
Then, this partition wall is formed of porous material. This
particulate filter is created by gathering tips of the partition
walls such that adjacent partition walls are brought into contact
with each other and baking them. Due to the fact that the partition
walls are baked in a state where they are in contact with each
other, the adjacent partition walls are bonded together at a
predetermined bonding strength. Further, according to the first
aspect of the invention, the end portion of the particulate filter
has a higher bonding strength than the predetermined bonding
strength.
[0008] The increased bonding strength can be achieved by a
plurality of measures. It is important that the predetermined
bonding strength is to be understood as the bonding strength which
is achievable by the baked contact surface of end portions of
adjacent wall plates bent towards each other.
[0009] A second aspect of the invention relates to a manufacturing
method of a particulate filter for collecting particulates in
exhaust gas. This method includes the steps of forming a
preliminary formed body having partition walls defining a path by
extruding porous material, closing the path of the preliminary
formed body by gathering an end portion of the partition wall of
the preliminary formed body so that tips of adjacent end portions
are in contact with each other, baking the preliminary formed body,
and reinforcing the closed portion in the path.
[0010] A third aspect of the invention relates to a manufacturing
method of the particulate filter for collecting particulates in
exhaust gas. This method includes the steps of forming a
preliminary formed body having partition walls defining a path by
extruding porous material, closing the path of the preliminary
formed body by gathering an end portion of the partition wall of
the preliminary formed body so that tips of adjacent end portions
are in contact with each other, baking the preliminary formed body,
and loading the end portion of the partition wall closing the path
of the preliminary formed body with a substance capable of
oxidizing particulates.
[0011] A fourth aspect of the invention relates to an exhaust gas
purifying apparatus having a particulate filter for collecting
particulates in exhaust gas. An end portion of a path of the
particulate filter includes a bonding portion bonded together at a
predetermined bonding strength when tips of adjacent partition
walls formed of porous material defining the path are brought in
contact and are baked. In the fourth aspect of the invention, an
average pore diameter of the bonding portion is smaller than an
average pore diameter of other partition wall than the end
portion.
[0012] A fifth aspect of the invention relates to an exhaust gas
purifying apparatus having a particulate filter for collecting
particulates in exhaust gas. In the fifth aspect of the invention,
adjacent partition walls are bonded together by baking over a
predetermined length from the tip of the partition wall made of
porous material defining a path of the particulate filter such that
the adjacent partition walls are in contact with each other and the
adjacent partition walls are boned in parallel with each other on a
bonded portion of the partition walls.
[0013] A sixth aspect of the invention relates to a particulate
filter for collecting particulates in exhaust gas. This particulate
filter includes a body portion formed with partition walls made of
porous material defining a path in which the exhaust gas flows, and
an end portion including a bonding portion bonded at a
predetermined bonding strength when tips of the adjacent partition
walls are in contact with each other and baked. In the sixth aspect
of the invention, the end portion has a higher strength than the
predetermined bonding strength.
[0014] A seventh aspect of the invention relates to a particulate
filter for collecting particulates in exhaust gas. This particulate
filter includes a body portion formed with partition walls made of
porous material defining a path in which the exhaust gas flows, and
an end portion including a bonding portion bonded at a
predetermined bonding strength when tips of the adjacent partition
walls are in contact with each other and baked. In the seventh
aspect of the invention., an average pore diameter of the bonding
portion is smaller than an average pore diameter of the partition
wall of the body portion.
[0015] A eighth aspect of the invention relates to a particulate
filter for collecting particulates in exhaust gas. This particulate
filter includes a body portion formed with partition walls made of
porous material defining a path in which the exhaust gas flows, and
a bonding portion baked with tips of the adjacent partition walls
being in contact with each other. In the eighth aspect of the
invention, the bonding portion is formed by bonding such that the
adjacent partition walls are in parallel with each other.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing and further objects, features and advantages
of the invention will become apparent from the following
description of preferred embodiments with reference to the
accompanying drawings, wherein like numerals are used to represent
like elements and wherein:
[0017] FIGS. 1A, 1B are diagrams showing a particulate filter
according to a first embodiment of the invention;
[0018] FIGS. 2A, 2B are diagrams showing an upstream end portion
and a downstream end portion of the particulate filter of the first
embodiment;
[0019] FIGS. 3A, 3B are diagrams showing an upstream end portion
and a downstream end portion of a conventional particulate
filter;
[0020] FIG. 4A is a front view showing a honeycomb structure;
[0021] FIG. 4B is a side view showing the honeycomb structure,
reinforcement member and die;
[0022] FIGS. 5A, 5B are diagrams showing the reinforcement member
in FIG. 4B;
[0023] FIGS. 6A, 6B are diagrams showing the die in FIG. 4B;
[0024] FIG. 7A is a diagram showing the particulate-filter of the
second embodiment;
[0025] FIG. 7B is a diagram showing the honeycomb structure and die
of the second embodiment;
[0026] FIGS. 8A, 8B are diagrams for explaining oxidation process
of particulates;
[0027] FIGS. 9A, 9C are diagrams for explaining deposition process
of particulates; and
[0028] FIG. 10 is a diagram showing the relation between the amount
of oxidation removable particulates and the temperature of the
particulate filter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] Hereinafter, the first embodiment of the invention will be
described with reference to the accompanying drawings. FIG. 1A is
an end face diagram of the particulate filter and FIG. 1B is a
longitudinal sectional view of the particulate filter. As shown in
FIGS. 1A, 1B, the particulate filter 22 has a honeycomb structure,
containing a plurality of exhaust gas paths extending in parallel.
The exhaust gas path constituted by an exhaust gas flow-in path 50
whose downstream end opening is closed by a tapered wall
(hereinafter referred to as downstream tapered wall) 52 and an
exhaust gas flow-out path 51 whose upstream end opening is closed
by a tapered wall (hereinafter referred to as upstream tapered
wall) 53. Namely, some part of the exhaust gas flow path (exhaust
gas flow path 50) is closed by the downstream tapered wall 52 at
the downstream end thereof, while the remaining exhaust gas flow
path (exhaust gas flow-out path 51) is closed by the upstream
tapered wall 53 at the upstream end thereof.
[0030] The downstream tapered wall 52 is formed by gathering and
connecting the downstream end partition portion of the partition
wall, which defines the exhaust gas flow-in path 50 of the
particulate filter 22. On the other hand, the upstream tapered wall
53 is formed by gathering and connecting the upstream end partition
portion of the partition wall, which defines the exhaust gas
flow-out path 51 of the particulate filter 22.
[0031] According to the present embodiment, the exhaust gas flow-in
path 50 and the exhaust gas flow-out path 51 are arranged
alternately through a thin partition wall 54. In other words, the
exhaust gas flow-in paths 50 and the exhaust gas flow-out paths 51
are constructed such that each exhaust gas flow-in path 50 is
surrounded by four exhaust gas flow-out paths 51 while each exhaust
gas flow-out path 51 is surrounded by four exhaust gas flow-in
paths 50. That is, one exhaust gas flow path (exhaust gas flow-in
path 50) of two adjacent exhaust gas flow paths, is closed
completely by the downstream tapered wall 52 at the downstream end
thereof while the other exhaust gas flow path (exhaust gas flow-out
path 51) is closed completely by the upstream tapered wall 53 at
the upstream end.
[0032] As shown in FIGS. 2A, 2B, reinforcement members 55, 56 are
attached to tips of these tapered walls 52, 53. These reinforcement
members 55, 56 are attached to the tapered walls 52, 53 so as to
cover at least the tips of, or the entire tapered walls 52, 53.
[0033] The particulate filter 22 is formed of, for example, porous
material such as cordierite. Thus, exhaust gas flowing into the
exhaust gas flow-in path 50 passes through the surrounding
partition wall 54 as indicated by an arrow in FIG. 1B and flows
into the adjacent exhaust gas flow-out path 51. Since the tapered
walls 52, 53 are a part of the partition wall 54, these tapered
walls 52, 53 are, of course, also formed of the same porous
material as the partition wall 54. Further, according to the
present embodiment, since the reinforcement members 55, 56 are also
formed of porous material, exhaust gas passes through the upstream
tapered wall 53 and the reinforcement member 56 as indicated by an
arrow in FIG. 2A and flows into the exhaust gas flow-out path 51
and as indicated by an arrow in FIG. 2B, passes through the
downstream tapered wall 52 and the reinforcement member 55 and
flows out.
[0034] The upstream tapered wall 53 is formed in a quadrangular
pyramid shape in which the sectional area of the exhaust gas
flow-out path 51 is gradually decreased as it approaches the
upstream. Of course, the reinforcement member 56, which is attached
so as to cover the upstream tapered wall 53, is also formed in a
quadrangular pyramid shape which becomes narrower as it approaches
the upstream. Thus, the upstream end of the exhaust gas flow-in
path 50, formed by four surrounding upstream tapered walls 53 has a
quadrangular pyramid shape in which the sectional area of the flow
path is gradually increased toward the upstream. As a result, as
compared to a case where an intake opening of the exhaust gas
flow-in path is formed as shown in FIG. 3A, exhaust gas flows into
the particulate filter more easily.
[0035] That is, in the particulate filter shown in FIG. 3A, the
upstream end of the exhaust gas flow-out path is closed by a plug
72. In this case, since part of exhaust gas collides with the plug
72 as indicated with a solid line, exhaust gas does not easily flow
into the exhaust gas flow-in path. As a result, pressure loss of
the particulate filter is increased. Further, since exhaust gas
flowing into the exhaust gas flow-in path from near the plug 72
becomes turbulent in the vicinity of the inlet as indicated with a
dotted line, it is more difficult for the exhaust gas to flow into
the exhaust gas flow-in path. As a result, pressure loss of the
particulate filter is further increased.
[0036] On the other hand, the particulate filter 22 of the present
embodiment allows exhaust gas to flow into the exhaust gas flow-in
path 50 without causing any turbulent flow in exhaust gas as shown
in FIG. 2A. Thus, according to the present embodiment, exhaust gas
can easily flow into the particulate filter 22. Therefore, the
pressure loss of the particulate filter is small.
[0037] In the particulate filter shown in FIGS. 3A, 3B,
particulates in exhaust gas are easily deposited on the upstream
end face of the plug 72 and the surface of the partition wall
nearby. The reason for this is that exhaust gas collides with the
plug 72 and exhaust gas becomes turbulent near the plug 72.
However, in the particulate filter 22 of the present embodiment,
the upstream end face which exhaust gas strongly collides with does
not exist, since the upstream tapered wall 53. Further, the
reinforcement member 56 are quadrangular pyramid and exhaust gas
does not become turbulent near the upstream end face. Therefore,
according to the present embodiment, a great number of particulates
are not deposited on the upstream end region of the particulate
filter 22, so that the pressure loss of the particulate filter 22
is suppressed.
[0038] On the other hand, the downstream tapered wall 52 of the
present embodiment is formed in a quadrangular pyramid shape such
that the sectional area of the flow path of the exhaust gas flow-in
path 50 is gradually decreased as it approaches the downstream. Of
course, the reinforcement member 55 attached so as to cover the
downstream tapered wall 52 is also formed to be accommodated in a
quadrangular pyramid shape which becomes narrower as it approaches
the downstream. Thus, the downstream end of the exhaust gas
flow-out path 51 formed by four surrounding downstream tapered
walls 52 expands in a quadrangular pyramid shape in which the
sectional area is gradually decreased as it approaches the
downstream. As a result, exhaust gas easily flows out of the
particulate filter as compared to a case where an exit opening of
the exhaust gas flow-out path is formed as shown in FIG. 3B.
[0039] That is, in the particulate filter shown in FIG. 3B, the
downstream end of the exhaust gas flow-in path is closed by a plug
70 and the exhaust gas flow-out path extends straight up to the
exit. In this case, part of exhaust gas flowing out of the exit
opening in the exhaust gas flow-out path flows along the downstream
end face, so that a turbulent flow 71 is formed in the vicinity of
the exit opening in the exhaust gas flow-out path. If a turbulent
flow is formed in this way, exhaust gas does not easily flow out of
the exhaust gas flow-out path.
[0040] On the other hand, in the particulate filter of the present
embodiment, as shown in FIG. 2B, no turbulent flow is formed in
exhaust gas, so that the exhaust gas can flow out of the exit
opening of the exhaust gas flow-out path 51. Thus, according to the
present embodiment, exhaust gas relatively easily flows out of the
particulate filter. Therefore, the pressure loss of the particulate
filter 22 is small.
[0041] In the meantime, the tapered walls 52, 53 and the
reinforcement members 55, 56 may be formed in any other form than
the quadrangular pyramid, for example, conical as long as it
becomes gradually narrower as it approaches outside of the
particulate filter 22.
[0042] Next, the reinforcement member of the first embodiment will
be described in detail. The tapered walls 52, 53 of the particulate
filter 22 of the above-described type are formed by gathering the
partition walls which define the paths in the honeycomb structure
made of porous material, that is, end portions of the partition
walls 54 so that tips thereof are in contact with each other and
baking the honeycomb structure. That is, the end portions of the
partition walls are baked, so that the end portions are bonded
together.
[0043] Actually, when the honeycomb structure is baked, a hole may
be made in the tip of the tapered walls 52, 53 depending on a case,
since the tips of the partition walls 54 are separated due to an
influence of thermal expansion of the end portions of the partition
walls. According to the present embodiment, since the basic
configuration of the particulate filter 22 is that the end portions
of the exhaust gas flow paths (exhaust gas flow-in path 50, exhaust
gas flow-out path 51) are closed completely by the tapered walls
52, 53, the hole is not made in the tip of the tapered walls 52, 53
like this.
[0044] According to the present embodiment, before the honeycomb
structure is baked, the reinforcement members 55, 56 are disposed
at the tips of the tapered walls 52, 53 and after that, the
honeycomb structure is baked. While the tapered walls 52, 53 are
formed by bonding together the tips of the separate partition walls
54, the reinforcement members 55, 56 are integrated members.
Therefore, they hold the tips of the partition walls 54, which form
the tapered walls 52, 53, so that they do not leave each other,
when the honeycomb structure is baked.
[0045] According to the present embodiment, the bonding strength of
the bonding region of the partition wall 54, which constructs the
tapered walls 52., 53, is increased, thereby preventing a hole from
being made in the tips of the tapered walls 52, 53.
[0046] The average pore diameter of each of the reinforcement
members 55, 56 is determined by the degree of increase of the
pressure loss of the entire particulate filter 22 when the
reinforcement members 55, 56 are attached to the tips of the
tapered walls 52, 53 and the extent of reinforcement necessary for
preventing any hole from being made in the tips of the tapered
walls 52, 53. That is, a reinforcement member having a larger
average pore diameter is used as the necessity of suppressing the
increase of the pressure loss is larger. Then, a reinforcement
member having a smaller average pore diameter is used as the level
of reinforcement needs to be increased. In the meantime, the
average pore diameter of the reinforcement member in the present
embodiment is smaller than the average pore diameter of the
partition wall 54.
[0047] As a modification of the first embodiment, the reinforcement
members 55, 56, particularly the tips thereof may be loaded with a
substance capable of oxidizing particulates in exhaust gas.
Consequently, the average pore diameter of the reinforcement
members 55, 56 becomes smaller than a case where they are loaded
with no substance capable of oxidizing particulates. Thus, even if
a hole is made in the tip of the tapered walls 52, 53 when the
honeycomb structure is baked, the average pore diameter of the tip
of the reinforcement members 55, 56 is small. Therefore, the
particulates in exhaust gas are prevented from flowing out of the
particulate filter without being collected by the particulate
filter.
[0048] Of course, if the hole in the tip of the tapered walls 52,
53 is closed by attaching the reinforcement members 55, 56 to the
tapered walls 52, 53 after the honeycomb structure is baked, at
least the object of the invention is achieved. In this case also,
particulates in exhaust gas can be prevented from flowing out of
the particulate filter 22 securely without being collected by the
reinforcement members 55, 56 by reducing the average pore diameter
of the reinforcement members 55, 56, which are allowed to carry a
substance capable of oxidizing the particulates.
[0049] In the meantime, it is important to construct the
particulate filter 22 so that the pressure loss is latently small
and keep the pressure loss from exceeding largely a latently
achievable value during use of the particulate filter 22, in
viewpoints of its performance.
[0050] That is, in case where an internal combustion engine is
provided with a particulate filter, operation control of the
internal combustion engine is so designed considering the latent
pressure loss of the particulate filter. Even if the particulate
filter is constructed so as to keep the pressure loss low, if the
pressure loss exceeds its latently achievable value during use, the
performance of the internal combustion engine is decreased.
[0051] Thus, according to the present embodiment, the partition
wall which defines the upstream end region of the exhaust gas flow
path in the particulate filter 22 is formed of a tapered wall and
further, the reinforcement member covering this partition wall is
formed also of a tapered member. Consequently, a turbulent flow is
prevented when exhaust gas flows into the exhaust gas flow path so
as to keep the pressure loss of the particulate filter 22 latently
low.
[0052] As described above, the partition wall which defines the
upstream end region of the exhaust gas flow path in the particulate
filter 22 is formed of the tapered wall and the reinforcement
member covering this partition wall is formed of the tapered
member. Therefore, particulates are not easily deposited on the
wall of such tapered reinforcement member. That is, the
particulates are prevented from being deposited on the wall of the
tapered reinforcement member to produce a turbulent flow in exhaust
gas flowing into the exhaust gas flow path during use of the
particulate filter 22. As a result, the pressure loss can be
prevented from being increased far beyond its latently achievable
value during use of the particulate filter 22.
[0053] As described above, particulates are not easily deposited on
the upstream reinforcement member 56 during use of the particulate
filter 22. However, the particulates can be deposited on the
upstream reinforcement member 56. In this case, the pressure loss
is increased during use of the particulate filter 22.
[0054] Thus, according to the above-described modification of the
embodiment of the invention, the upstream reinforcement member 56
is loaded with a substance capable of oxidizing and removing
particulates so as to oxidize and remove the particulates deposited
on the upstream reinforcement member 56. As a result, since
particulates collected by the upstream reinforcement member 56 are
continuously oxidized and removed, no great number of the
particulates are deposited on the upstream reinforcement member 56.
Therefore, the pressure loss can be kept low during use of the
particulate filter 22.
[0055] According to the present embodiment and its modification, a
problem inherent to the structure of closing the exhaust gas
flow-out path 51 that is, a problem that the pressure loss deviates
from its achievable value during use of the particulate filter can
be avoided by the upstream tapered wall 53 and the tapered
reinforcement member 56 made of porous material in order to reduce
the pressure loss of the particulate filter 22 latently.
[0056] According to the modification of the present embodiment, a
substance capable of oxidizing particulates is loaded on the entire
particulate filter 22, that is, not only on the upstream
reinforcement member 56, but also on the upstream tapered wall 53,
the partition wall 54, the downstream tapered wall 52 and the
downstream reinforcement member 55. Further, a substance capable of
oxidizing particulates is carried by not only the walls of the
upstream reinforcement member 56, the upstream tapered wall 52, the
partition wall 54, the downstream tapered wall 52, and the
downstream reinforcement member 55, but also these pore walls
inside. According to the modification of the present embodiment,
the amount by a unit volume of the substance capable of oxidizing
particulates loaded on the upstream reinforcement member 56 and the
upstream tapered wall 53 is larger than the amount by a unit volume
of the substance capable of oxidizing particulates loaded on the
partition wall 54, the downstream tapered wall 52 and the
downstream reinforcement member 55.
[0057] According to the present embodiment, although the upstream
end opening and the downstream end opening of the particulate
filter are closed completely, the concept of the invention can be
applied to the particulate filter in which only any one of the
upstream end opening and the downstream end opening is completely
closed.
[0058] Next, a manufacturing method of a particulate filter of the
present embodiment will be described briefly. First, a cylindrical
honeycomb structure 80 is formed of porous material such as
cordierite by extrusion as a preliminary formed body as shown in
FIGS. 4A, 4B. The honeycomb structure 80 has a plurality of exhaust
gas flow paths each having a square section. Part of these exhaust
gas flow paths serves as the exhaust gas flow-in paths 50 in the
particulate filter 22, while the remaining exhaust gas flow paths
serves as the exhaust gas flow-out paths 51 in the particulate
filter 22.
[0059] Next, a reinforcement member 81 made of porous material is
disposed on each end face of the honeycomb structure 80 as shown in
FIG. 4B. As shown in FIG. 5A, each reinforcement member 81 has a
disc portion 82 fitting to a circular end face of the honeycomb
structure 80. As shown in FIG. 5A, a plurality of leg portions 83
extends vertically from the disc portion 82. As shown in FIG. 5B,
each of these leg portions 82 has a square of the square tube.
[0060] When the reinforcement member 81 is disposed on the end face
upstream of the honeycomb structure 80, each leg portion 83 is
accommodated in the exhaust gas flow-in path 50. On the other hand,
when the reinforcement member 81 is disposed on the end face
downstream of the honeycomb structure 80, each leg portion 83 is
accommodated in the exhaust gas flow-out path 51. FIG. 5A indicates
a sectional view taken along the lines 5A-5A in FIG. 5B.
[0061] Next, a die 90 shown in FIG. 6 is pressed to the end face of
the honeycomb structure 80 together with the reinforcement member
81. The die 90 is pressed to one end face of the honeycomb
structure 80 and then to the other end face. Of course, it is
permissible to prepare two dies 90 and press them to each end face
of the honeycomb structure 80 at the same time.
[0062] As shown in FIG. 6A, the die 90 has a plurality of
quadrangular pyramid shaped protrusions 91. FIG. 6B shows a
protrusion 91. The die 90 is pressed to an end face of the
honeycomb structure 80 together with the reinforcement member 81
such that the protrusions 91 are inserted into each predetermined
exhaust gas flow path. When the protrusions 91 of the die 90 are
inserted into predetermined exhaust gas flow paths, the disc
portion 82 of the reinforcement member 81 is broken by these
protrusions 91. If the die 90 is moved further toward the end face
of the honeycomb structure 80, the disc portion 82 and the leg
portion 83 of the reinforcement member 81 are gathered. At the same
time, the partition walls 54, which form a predetermined exhaust
gas flow path, are gathered so as to form the tapered walls 52, 53.
Consequently, the predetermined exhaust gas flow paths are closed
completely by the tapered walls 52, 53 covered by the reinforcement
members 55, 56.
[0063] Next, the honeycomb structure 80 is dried, and then, the
honeycomb structure 80 is baked. Next, the honeycomb structure is
loaded with a substance capable of oxidizing particulates. As a
result, the particulate filter 22 is formed.
[0064] As described above, the end portion of the particulate
filter 22 is closed by the tapered walls 52, 53 composed of the
same porous material as the partition wall 54. Therefore, the
exhaust gas flow path (exhaust gas flow-in path 50, exhaust gas
flow-out path 51) of the particulate filter 22 can be closed by the
same material as the partition wall 54 according to such a simple
method of pressing the die 90 against the end face of the honeycomb
structure 80 as described above.
[0065] The step of disposing the reinforcement member 81 on the end
face of the honeycomb structure 80 and pressing the die 90 against
the end face of the honeycomb structure 80 may be executed after
the honeycomb structure 80 is dried. Alternatively, it is
permissible to soften the end portion of the honeycomb structure 80
after the honeycomb structure 80 is baked, then dispose the
reinforcement member 81 on the end face of the honeycomb structure
80 and press the die 90 to the softened end portion. In this case,
the end portion of the honeycomb structure 80 is baked again after
that.
[0066] As a second modification of the present embodiment, a
quadrangular pyramid shaped reinforcement member composed of porous
material may be disposed directly on the tapered wall after the
honeycomb structure 80 is baked.
[0067] Although the leg portion 83 of the reinforcement member 81
is means for positioning securely and holding the reinforcement
member 81 on the honeycomb structure, the leg portion 83 may be
eliminated if other means for achieving this is provided.
[0068] Next, the particulate filter of the second embodiment will
be described. According to the second embodiment, as shown in FIG.
7A, the end portions of the partition walls 54 are bonded over a
predetermined length from the tip so as to form extended portions
57, 58. In the downstream region of the particulate filter 22, the
end portions downstream of the partition walls 54 are bonded
together with adjacent parallel portions over a predetermined
length toward the upstream from the tip so as to form an extended
portion 57. On the other hand, in the upstream region of the
particulate filter 22, the end portions upstream of the partition
walls 54 are bonded together over a predetermined length toward
downstream from the tip so as to form an extended portion 58.
[0069] As shown in FIG. 7B, these extended portions 57, 58 are
formed by pressing the die 90 having the quadrangular pyramid
shaped protrusions 91 and a rectangular portion 92 adjacent to this
protrusion 91 and further containing a groove 93 between these
rectangular portions 92 against each end face of the honeycomb
structure 80.
[0070] Thus, according to the present embodiment, the bonding
region of the end portions bonded together of the partition walls
54 in the end portion region of the particulate filter 22 is larger
than the bonding region when only the tips of the partition walls
are bonded together. For this reason, the bonding strength of the
partition walls in the end portion region of the particulate filter
22 of the present embodiment is higher than the bonding strength
when only the tips of the partition walls are bonded together.
[0071] As a modification of the second embodiment, the bonding
strength of the end portions of the partition walls 54 which
construct the extended portions 57 can be increased by loading on
the extended portions 57, 58 with a substance capable of oxidizing
the particulates. Of course, the entire particulate filter 22 may
be loaded with the aforementioned substance capable of oxidizing
the particulates.
[0072] In case of loading the honeycomb structure 80 with a
substance capable of oxidizing particulates according to the
present embodiment, the step of loading the honeycomb structure 80
with this substance capable of oxidizing particulate is carried out
after the step of baking the honeycomb structure 80.
[0073] Next, the particulate filter 22 of the third embodiment will
be described. The structure and operation of the particulate filter
22 of the third embodiment are the same as those of the first
embodiment except the items described below. Therefore, description
about the same structure and operation as the first embodiment is
omitted.
[0074] According to the third embodiment, the reinforcement members
55, 56 of the first embodiment are omitted. According to the third
embodiment, in place of them, the average pore diameter of the end
portions of the partition walls 54 to be bonded together so as to
form the tapered walls 52, 53 is set smaller than the average pore
diameter of the partition wall 54.
[0075] According to the present embodiment, assuming a case where
the areas of the end portions of the partition walls 54 bonded
together are equal, the area of the end portions of the partition
walls 54 substantially being in contact with each other is larger
than a bonding region of tips in an end portions having the same
average pore diameter as the partition wall 54. Therefore, the
bonding strength of the partition walls in an end portion region of
the particulate filter 22 of the present embodiment is higher than
the bonding strength of a case where the end portions having the
same pore density as the partition wall 54 are bonded to each
other.
[0076] According to the present embodiment also, it is of course
permissible to increase the bonding strength of the tips of the
partition walls 54 by loading the end portions of the partition
walls 54 bonded together with a substance capable of oxidizing the
particulates.
[0077] According to the present embodiment, a step of reducing the
average pore diameter of the end portions of the partition walls 54
to be bonded together may be executed between a step of closing the
exhaust gas flow path in the honeycomb structure 80 with the end
portions and a step of baking the honeycomb structure 80. A step of
loading the honeycomb structure 80 with a substance capable of
oxidizing the particulates may be carried out after a step of
baking the honeycomb structure 80.
[0078] Next, a particulate filter of the fourth embodiment will be
described. According to the present embodiment, the bonding
strength of the tips of the end portions of the partition walls to
be bondeded together is incresed in order to prevent the tips of
the end portions of the partition walls, which compose the tapered
wall, from being separated and producing a hole when the honeycomb
structure is baked. That is, an object of the above-described
embodiment is to prevent a hole from being made in the tip of the
tapered wall.
[0079] The object of the fourth embodiment is to prevent exhaust
gas from flowing out of a hole in the tip of the tapered wall by
closing the hole made in the tip of the tapered wall in a simple
way. More specifically, according to the fourth embodiment, a hole
made in the tip of each of the tapered walls 52, 53 is closed by
loading the tips of the tapered walls 52, 53 with a substance
capable of oxidizing the particulates after the honeycomb structure
80 is baked.
[0080] Finally, a substance capable of oxidizing particulates
loaded on the particulate filter 22 will be described in detail.
According to the above-described embodiment, a carrier layer made
of alumina and the like is formed on the peripheral wall face of
each exhaust gas flow-in path 50 and the inside of the peripheral
walls and each exhaust gas flow-out path 51 i.e., both side
surfaces and inside of each partition wall 54, both side surface
and inside of the tapered walls 52, 53, and both side surfaces and
inside of a reinforcement member if it is provided. Then, this
carrier is loaded with noble metal catalyst and active oxygen
discharging agent which if excessive oxygen exists, takes in and
retains oxygen and if the concentration of oxygen decreases,
releases the retained oxygen in the form of active oxygen.
According to the above-described embodiments, this the substance
capable of oxidizing the particulates is the active oxygen
discharging agent.
[0081] According to the above-described embodiments, platinum Pt is
used as noble metal catalyst and as active oxygen discharging
agent, at least one selected from alkaline metals such as potassium
K, sodium Na, lithium Li, cesium Cs, rubidium Rb, alkaline earth
metals such as barium Ba, calcium Ca, strontium Sr, rare earth
elements such as lanthanum La, yttrium Y, cerium Ce, transition
metal such as iron Fe, and carbon group element such as tin Sn is
employed.
[0082] It is preferable to use alkaline metal or alkaline earth
metal ensuring a higher ionization tendency than calcium Ca, such
as potassium K, lithium Li, cesium Cs, rubidium Rb, barium Ba, and
strontium Sr.
[0083] Next, an action for removing particulates from exhaust gas
by means of the particulate filter 22 will be described about a
case where platinum Pt and potassium K are loaded on a carrier. The
same particulate removal action is carried out if other noble
metals, alkaline metals, alkaline earth metals, rare earth
elements, or transition metals are employed.
[0084] For example, if exhaust gas flowing into the particulate
filter 22 is gas emitted from a compression ignition type internal
combustion engine which burns with excessive air, exhaust gas
flowing into the particulate filter 22 contains a great amount of
excessive air. That is, if the ratio between air and fuel supplied
into an intake air path and a fuel combustion chamber is referred
to as air-fuel ratio of exhaust gas, the air-fuel ratio of exhaust
gas in the compression ignition type internal combustion engine is
lean. Further, since NO is generated in the fuel combustion chamber
of the compression ignition type internal combustion engine, NO is
contained in exhaust gas. Added to that, sulfur component S is
contained in fuel and this sulfur component S reacts with oxygen in
the fuel combustion chamber so as to produce SO.sub.2. Therefore,
SO.sub.2 is contained in exhaust gas. Thus, exhaust gas containing
excessive oxygen, NO, and SO.sub.2 flows into the exhaust gas
flow-in path 50 of the particulate filter 22.
[0085] FIGS. 8A, 8B show schematically an enlarged diagram of the
surface of a carrier formed on an inner peripheral face of the
exhaust gas flow-in path 50. In FIGS. 8A, 8B, the particulate 60 is
a particulate of platinum Pt and the active oxygen discharging
agent 61 contains potassium K.
[0086] As described above, since exhaust gas contains a large
amount of excessive oxygen, if exhaust gas flows into the exhaust
gas flow-in path 50 in the particulate filter 22, oxygen O.sub.2
adheres to the surface of platinum Pt 60 in the form of O.sub.2 or
O.sup.2-. On the other hand, NO in exhaust gas reacts with
O.sub.2.sup.- or O.sup.2- on the surface of platinum Pt 60 so as to
produce NO.sub.2 (2NO+O.sub.2.fwdarw.2NO.sub.2) Part of NO.sub.2
generated next is oxidized on platinum Pt 60 and absorbed into the
active oxygen discharging agent 61 and then combined with potassium
K so that it is diffused into the active oxygen discharging agent
61 in the form of nitrate ion NO.sub.3.sup.- and produces potassium
nitrate KNO.sub.3.
[0087] On the other hand, SO.sub.2 is contained in exhaust gas as
described above and this SO.sub.2 is absorbed in the active oxygen
discharging agent 61 in the same mechanism as NO. That is, as
described above, oxygen O.sub.2 adheres to the surface of platinum
Pt 60 in the form of O.sub.2.sup.- or O.sup.2- and SO.sub.2 in
exhaust gas reacts with O.sub.2.sup.- or O.sup.2- on the surface of
platinum Pt 60 and turns to SO.sub.3. Part of SO.sub.3 generated
next is oxidized on platinum Pt 60 and absorbed into the active
oxygen discharging agent 61, so that it is combined with potassium
K and diffused into the active oxygen discharging agent 61 in the
form of sulfuric ion SO.sub.4.sup.2- so as to produce potassium
sulfate K.sub.2SO.sub.4. As a result, potassium nitrate KNO.sub.3
and potassium sulfate K.sub.2SO.sub.4 are generated.
[0088] On the other hand, particulates composed of mainly carbon C
are generated in a combustion chamber and therefore, these
particulates are contained in exhaust gas. When exhaust gas flows
in the exhaust gas flow-in path 50 in the particulate filter 22 or
flows from the exhaust gas flow-in path 50 to the exhaust gas
flow-out path 51, these particulates 62 contained in exhaust gas
come into contact with and adhere to the surface of a carrier, for
example, the active oxygen discharging agent 61.
[0089] If particulates 62 adhere to the surface of the active
oxygen discharging agent 61, the concentration of oxygen on the
contact face between the particulates 62 and the active oxygen
discharging agent 61 decreases. If the concentration of oxygen
decreases, a difference in the concentration of oxygen occurs
between the contact face of the active oxygen discharging agent 61
and the active oxygen discharging agent 61 whose concentration of
oxygen is higher, so that oxygen in the active oxygen discharging
agent 61 tries to move toward the contact face between the
particulates 62 and the active oxygen discharging agent 61. As a
result, potassium nitrate KNO.sub.3 formed in the active oxygen
discharging agent 61 is decomposed to potassium K, oxygen O and NO.
Oxygen 0 moves toward the contact face between the particulates 62
and the active oxygen discharging agent 61, while NO is discharged
out of the active oxygen discharging agent 61. NO discharged out is
oxidized on platinum Pt 60 downstream and absorbed into the active
oxygen discharging agent 61 again.
[0090] In addition, potassium sulfate K.sub.2SO.sub.4 formed in the
active oxygen discharging agent 61 is decomposed to potassium K,
oxygen O, and SO.sub.2. Oxygen 0 moves toward the contact face
between the particulates 62 and the active oxygen discharging agent
61, while SO.sub.2 is discharged out of the active oxygen
discharging agent 61. SO.sub.2 discharged out is oxidized on
platinum Pt 60 downstream and absorbed into the active oxygen
discharging agent 61 again. However, since potassium sulfate
K.sub.2SO.sub.4 is stable and difficult to decompose, potassium
sulfate K.sub.2SO.sub.4 does not easily emit active oxygen than
potassium nitrate KNO.sub.3.
[0091] The active oxygen discharging agent 61 generates and
discharges active oxygen also in a reaction process with oxygen
when NO.sub.x is absorbed in the form of nitrate ion NO.sub.3.sup.-
as described above. Likewise, the active oxygen discharging agent
61 generates and discharges active oxygen in a reaction process
with oxygen when SO.sub.2 is absorbed in the form of sulfate ion
SO.sub.4.sup.2-.
[0092] Oxygen O that moves toward the contact face between the
particulates 62 and the active oxygen discharging agent 61 is
oxygen which is generated by decomposing such compound as potassium
nitrate KNO.sub.3; potassium sulfate K.sub.2SO.sub.4. Oxygen O
generated by decomposing the compound has a high energy and an
extremely high activity. Thus, oxygen which moves toward the
contact face between the particulates 62 and the active oxygen
discharging agent 61 acts as active oxygen O. Likewise, oxygen
generated in a reaction process between NO.sub.x and oxygen in the
active oxygen discharging agent 61 or in a reaction process between
SO.sub.2 and oxygen acts as active oxygen. If the active oxygen O
are in contact with the particulates 62, the particulates 62 are
oxidized without any luminous flame in a short time (several
seconds to several tens minutes), so that the particulates 62 are
vanished completely. Therefore, the particulates 62 are hardly
deposited on the particulate filter 22.
[0093] Some type of the particulate filter is heated in red and
burns the particulates with flame when particulates deposited in
layers on the particulate filter are burned. The combustion with a
flame does not continue unless a high temperature is kept.
Therefore, the temperature of the particulate filter has to be kept
high in order to continue combustion with the flame.
[0094] According to the embodiment of the invention, the
particulates 62 are oxidized without any luminous flame as
described above, and the surface of the particulate filter 22 is
not heated in red. In other words, according to the embodiment of
the invention, the particulates 62 are oxidized and removed under a
relatively lower temperatures as compared to combustion with flame.
Therefore, the particulate removal action by oxidation of
particulates 62 without any luminous flame according to the
embodiment of the invention is completely different from the
particulate removal action by combustion with flame.
[0095] Since platinum Pt 60 and active oxygen discharging agent 61
are activated more as the temperature of the particulate filter 22
is increased, the amount of oxidation removable particulates
without luminous flame per unit time on the particulate filter 22
is increased as the temperature of the particulate filter 22 is
increased.
[0096] A solid line in FIG. 10 indicates the amount G of oxidation
removable particulates without luminous flame per unit time. The
horizontal axis in FIG. 10 indicates the temperature TF of the
particulate filter 22. Hereafter, the amount of particulates
flowing into the particulate filter 22 per unit time is referred to
as flow-in particulate amount M. If this flow-in particulate amount
M is smaller than the oxidation removable particulates G i.e.,
within a region I in FIG. 10, all particulates flowing into the
particulate filter 22 being contact with the particulate filter 22
are oxidized without any luminous flame on the particulate filter
22 in a short time (several seconds to several ten minutes) and
removed.
[0097] Contrary to this, if the flow-in particulate amount M is
larger than the oxidation removable particulate amount G i.e.,
within a region H in FIG. 10, the amount of active oxygen is not
enough for oxidizing all particulates. FIGS. 9A to 9C show the
state of oxidation of particulates in such a case. That is, if the
amount of active oxygen is short for oxidizing all particulates and
the particulates 62 adhere to the active oxygen discharging agent
61 as shown in FIG. 9A, only part of the particulates 62 is
oxidized, while part of the particulates which is not oxidized
sufficiently remains on the carrier. If the state in which the
active oxygen amount is not enough continues, particulates which
are not sufficiently oxidized remain on the carrier successively,
so that as shown in FIG. 9B, the surface of the carrier is covered
with remaining particulate portion 63.
[0098] If the surface of the carrier is covered with the remaining
particulate portion 63, oxidation of NO and SO.sub.2 by platinum Pt
60 and discharge of active oxygen by the active oxygen discharging
agent 61 are eliminated, so that the remaining particulate portion
63 is left without being oxidized and slightly after, other
particulates are deposited successively on the remaining
particulate portion 63 as shown in FIG. 9C. That is, the
particulates are deposited in layers.
[0099] If the particulates are deposited in layers, the
particulates 64 are never oxidized by active oxygen 0 and
therefore, other particulates are deposited successively on the
particulates 64. That is, if the state in which the flow-in
particulate amount M is larger than the oxidation removable
particulate amount G is continued, particulates are deposited in
layers on the particulate filter 22 and the deposited particulates
cannot be ignited and burnt until the temperature of exhaust gas is
raised high or the temperature of the particulate filter 22 is
raised high.
[0100] In the region I of FIG. 10, particulates are oxidized on the
particulate filter 22 without any luminous flame in a short time
and in the region II of FIG. 10, particulates are deposited in
layers on the particulate filter 22. Thus, the flow-in particulate
amount M always needs to be smaller than the oxidation remove
particulate amount G for the particulates not to be deposited in
layers on the particulate filter 22.
[0101] As evident from FIG. 10, the particulate filter 22 used in
the embodiments of the invention is capable of oxidizing
particulates even if the temperature TF of the particulate filter
22 is quite low. Thus, the flow-in particulate amount M and the
temperature TF of the particulate filter 22 are kept so that the
flow-in particulate amount M is always smaller than the oxidation
removable particulate amount G.
[0102] If the flow-in particulate amount M is always smaller than
the oxidation removable particulate amount G, few particulates are
deposited on the particulate filter 22, so that the back pressure
is increased little.
[0103] On the other hand, if particulates are deposited in layers
on the particulate filter 22 as described above, even if the
flow-in particulate amount M becomes smaller than the oxidation
removable particulate amount G, it is difficult to oxidize the
particulates with active oxygen 0. That is, if the flow-in
particulate amount M becomes smaller than the oxidation removable
particulate amount G when the particulates are deposited only below
a predetermined level, this remaining particulate portion is
oxidized without any luminous flame by the active oxygen O and
removed.
[0104] If a case in which the particulate filter 22 is disposed in
the exhaust gas path of an internal combustion engine and actually
employed is considered, fuel and lubricant contain calcium Ca and
therefore, calcium Ca is contained in exhaust gas. If SO.sub.3
exists, this calcium Ca generates calcium sulfate CaSO.sub.4. This
calcium sulfate CaSO.sub.4 is solid, which is not thermally
decomposed even at a high temperature. Thus, if calcium sulfate
CaSO.sub.4 is generated and the pore in the particulate filter 22
is closed by this calcium sulfate CaSO.sub.4, exhaust gas does not
easily flow in the particulate filter 22.
[0105] In this case, if alkaline metal or alkaline earth metal
having a higher ionization tendency than calcium Ca, for example,
potassium K is employed as the active oxygen discharging agent 61,
SO.sub.3 diffused in the active oxygen discharging agent 61 is
combined with potassium K so as to form potassium sulfate
K.sub.2SO.sub.4. Calcium Ca passes through the partition wall 54 of
the particulate filter 22 without being combined with SO.sub.3 and
flows out into the exhaust gas flow-out path 51. Thus, the pores in
the particulate filter 22 are never clogged. Therefore, preferably,
as the active oxygen discharging agent 61, alkaline metal or
alkaline earth metal having a higher ionization tendency than
calcium Ca i.e., potassium K, lithium Li, cesium Cs, rubidium Rb,
barium Ba, or strontium Sr is employed.
[0106] The embodiments of the invention can be applied to a case
where only noble metal such as platinum Pt 60 is loaded on the
layer of a carrier formed on both side faces of the particulate
filter 22. However, in this case, the solid line indicating the
oxidation removable particulate amount G is moved slightly to the
right as compared to the solid line shown in FIG. 10. In this case,
active oxygen is discharged from NO.sub.2 or SO.sub.3 retained on
the surface of the platinum Pt 60.
[0107] Further, it is permissible to employ a catalyst, which
absorbs NO.sub.2 or SO.sub.3 as the active oxygen discharging agent
and can discharge active oxygen from these absorbed NO.sub.2 or
SO.sub.3.
* * * * *